While suitable for detecting any liquid level, the liquid-level indicating probe of the invention is particularly adapted for use in sensing the level of corrosive liquids in a tank or other container.
As is known, thermistors are semiconductors whose resistance varies as a function of temperature. In the past, thermistor-type liquid level detectors have been provided wherein several thermistors are spaced along the height of a tank. Each thermistor is supplied with a small electrical current which heats it. When the thermistor is immersed in a liquid, its temperature is different than when it is exposed to air due to a difference in the heat dissipation effect. As a result, the resistance of the thermistor undergoes a corresponding variation, depending upon whether it is immersed or exposed to the atmosphere. Assuming, therefore, that a number of thermistors are spaced along the height of a tank, those thermistors which are immersed will exhibit a different resistance value than those above the liquid level bath. This variation in resistance can be detected with suitable electrical circuitry to determine the location of the liquid level within the tank. Typical examples of such thermistor liquid-level probes are shown, for example, in U.S. Pat. Nos. 3,432,840 and 3,955,416.